Composite structures are often structurally tested during design and/or manufacture of the composite structure, as well as once the composite structure is in use. For example, a mechanical apparatus may be used to bend the composite structure, periodically flex the composite structure, and/or limit load the composite structure in order to mechanically test the composite structure. In some cases, this may be performed during design and manufacture of a composite structure, such as to analyze fatigue fault initiation and growth within the composite structure.
Fixed non-destructive inspection (NDI) sensors may be attached to test piece, or test specimen, (e.g., a composite structure being tested) to detect any indications of stress or damage during the testing. Often, single point ultrasonic transducers are mounted on the test specimen in locations where finite element analysis indicates they will be most useful. However, these fixed sensors tend to only collect data in the areas under the sensors and thus cannot monitor the entire composite structure. If indications appear in unexpected locations, data density in those unexpected locations may be sparse or non-existent. If unexpected indications occur, technicians may be required to stop the test and reposition the fixed sensors in order to obtain data from different locations on the structure. Oftentimes this may result in invalidating the entire test (and therefore having to restart it from the beginning), if, for example, the fixed sensors cannot be safely removed and repositioned without relieving the test stress placed on the composite structure.
Current non-contact testing methods, such as optical, IR thermography, and digital image correlation, are limited by their depth of penetration and often cannot provide adequate information. Other methods, such as microwave- and radar-based testing methods, often are limited by their low-resolution capabilities. Some X-ray-based testing methods may provide high resolution but are not well-suited for composite structures, especially large composite parts. Additionally, these conventional testing systems tend to be large and difficult to move.
Laser ultrasonic non-destructive inspection systems are also utilized to verify the structural integrity of composite materials. In this process, a pulsed generation laser is directed at the surface of the test specimen, with the beam being absorbed into a shallow volume of the material (e.g., the top 10-100 microns of the test specimen). The rapid absorption of the pulse laser energy creates a localized heating, which results in expansion of the material (referred to as thermo-elastic expansion), inducing a stress wave. These waves interrogate a feature of interest in the interior or surface of the test specimen, and then propagate to the surface position of a detection laser beam. The resulting surface displacement is measured with a laser ultrasonic receiver. The measured signal is then processed to yield and display the desired information. However, these conventional laser inspection systems have limited capability for real-time structural integrity monitoring and damage characterization, which may be due to limited resolution, needing a direct line of sight to the test piece, and/or having a limited thickness through which the laser inspection systems may be useful for scanning.